The present invention relates to an organic electron-conducting layer having at least one dopant for increasing the n-conductivity of the organic layer, characterized in that the dopant is selected from the group of the salts of the cyclopentadiene compounds.
Provision of effective organic electronic components in line with market demand requires not only the selection of highly efficient individual components but also the use of production technologies which enable inexpensive mass production. This applies especially to organic components, for example organic light-emitting diodes (OLEDs), the structure of which is shown in schematic form in FIG. 1, organic solar cells, shown in schematic form in FIG. 2, and organic field-effect transistors, shown in FIG. 3, the composition of which in the last few years has been subject to change directed to ever higher performance.
A key point in the development work for increasing efficiency is the establishment of a higher component quality, which can be achieved substantially through an increase in the charge carrier mobility and density within the organic electronic transport layers used.
There are basically two different approaches pursued in organic electronics to increase the electron conductivity. Firstly, an increase in the charge carrier injection can result from insertion of an intermediate layer between the cathode and electron transport layer. Secondly, an increase in the charge carrier density can be achieved by the n-doping of electrically conductive organic matrix materials with suitable donors. In this case, the matrix material is deposited as a layer together with the dopant either by co-sublimation from the gas phase or from a liquid phase.
For the former method, thin salt layers of LiF, CsF or, in the more recent literature, cesium carbonate are often used, these lowering the work function of the electrons. The properties and effects of cesium carbonate are described, for example, by Huang, Jinsong et al., Adv. Funct. Mater. 2007, 00, 1-8; Wu, Chih-I et al., APPLIED PHYSICS LETTERS 88, 152104 (2006) and Xiong, Tao et al., APPLIED PHYSICS LETTERS 92, 263305 (2008). These intermediate layers significantly improve electron transport, but this improvement is insufficient for high-efficiency layers.
For doping of electronic transport layers, it is generally the case that substances having a HOMO (highest occupied molecular orbital) above the LUMO (lowest unoccupied molecular orbital) of the matrix material are used. This is a prerequisite for transfer of an electron from the dopant to the matrix material and thus for an increase in its conductivity. In addition, preference is further given to introducing substances whose valence electrons have very low work functions or ionization energies. This too can facilitate the electron release of the dopant and thus increase the layer conductivity.
A way of doping organic semiconductor materials proposed, for example, by DE 103 38 406 A1 is that of electrically uncharged compounds having specific geometry, with elimination of hydrogen, carbon oxide, nitrogen or hydroxy radicals after the dopant has been mixed into the organic semiconductor material, and transfer of at least one electron to or from the semiconductor material. These compounds are characterized in that the dopant used is an uncharged organic compound. Example compounds mentioned are substituted and unsubstituted homo- or heterocycles which form an aromatic 6-π system through release or acceptance of an electron. A disadvantage of this solution is that the conditions on the matrix material for elimination of a hydrogen radical from the dopant are very restrictive, and are fulfilled extremely rarely with the matrix materials commonly used in organic electronics. In addition, for example, the hydrogen radicals that form are extremely reactive, which can lead to uncontrolled reactions and ultimately to damage to the layer or the component.
U.S. Pat. No. 5,922,396 A describes a method for producing a multilayer structure for organic electronics, containing both n(electron)-conductive and p(hole)-conductive layers. The multilayer construct is obtained inter alia by sublimation of organic metal complexes which form free radicals in the gas phase and are then deposited in this form as an electron-conducting layer.